1. Field of the Invention
The invention relates generally to reflectometer apparatus which detects RF signals reflected by an electrical load to which RF signals are transmitted. More specifically, the invention concerns an RF reflectometer which includes self-balancing means operative to automatically achieve and maintain a balanced or null output condition in response to variations in the reflected signals due to long term basal variations in the impedance of the load while simultaneously providing a detectable output in response to rapid variations in said signals, for example due to intentional modulation of the load impedance.
2. Description of the Prior Art
It is generally known in the field of RF signal transmission that when the output impedance of an RF source matches the input impedance of a load, the efficiency of power transfer between the source and load is maximized. Theoretically, when the source and load impedances are exactly matched, the load absorbs 100% of the signal transmitted by the source. However, when the load impedance varies from the source impedance, the load does not absorb the entire signal and "reflects" a portion of the signal back to the source.
By definition, impedance has a real component and an imaginary component. The RF impedance of the load can vary over time for any number of reasons. Such variations may occur in either the real or imaginary component of the impedance, or both.
Generally, a mismatch between the real components of the source and load impedance results in the load reflecting a portion of the transmitted signal in phase with the transmitted signal. This is the real component of the reflected signal. A mismatch between the imaginary components of the source and load impedances results in the load reflecting a portion of the transmitted signal back to the source with a phase variation from the transmitted signal. This is the imaginary component of the reflected signal. By definition, the real and imaginary components of the reflected signal are 90 degrees out of phase or in "phase quadrature."
The value of forward RF power from an RF source to a load gives an indication of the strength of the signal being transmitted to the load, and is useful in adjusting the output power level of the source to a desired level. However, forward power provides no information concerning the sign or magnitude of variations of the load impedance from the source impedance. The value of the reflected RF signal on the other hand contains information concerning both the sign and magnitude of mismatch between the source and load impedance.
Circuits and instruments which are responsive to forward RF power have been developed for use in RF communications systems to monitor and adjust the power output of RF transmitters in response to variations in load impedance, but without providing any information about or any detectable output indicative of the variations. See, for example, the devices described in Mitama U.S. Pat. No. 4,392,245 and Wheeler U.S. Pat. No. 3,717,811.
Circuits and instruments that detect the RF signal reflected by a load have also been developed for a variety of RF communications applications. Such devices have been employed to assist in matching source and load impedances to optimize long range transmission of RF signals between RF transmitters and receivers. See, for example, the automatic antenna coupler disclosed by Smolka U.S. Pat. No. 3,919,644; the automatic impedance matching system disclosed in Kuecken U.S. Pat. No. 3,601,717; and the automatic antenna tuning device disclosed in Ludvigson et al. U.S. Pat. No. 3,117,279. Such devices have also been employed to measure and monitor certain parameters of RF communications systems such as standing wave ratio (SWR), reflection coefficient, forward power level, and antenna impedance. See, for example, the coupling apparatus disclosed in Wheeler U.S. Pat. No. 3,717,811 (FIG. 2); the RF impedance bridge disclosed in Shekel U.S. Pat. No. 3,800,218; the transmitter test instrument disclosed in Schwartz U.S. Pat. No. 4,096,441; the forward power and antenna impedance measuring apparatus disclosed in Redlich U.S. Pat. No. 4,409,544; and the in-line wattmeter disclosed in Beaudry U.S. Pat. No. 3,678,381.
The known circuits and instruments are typically designed to operate with a characteristic fixed impedance Z.sub.o. In RF communications applications, this characteristic impedance is commonly 50 or 75 ohms with no reactive component, although in some instances the characteristic impedance may be selected to match the optimum load impedance for the RF source. Thus, it is generally necessary to manually calibrate known circuits and instruments using a load having the fixed characteristic impedance value before using them. In the case of RF power and impedance bridges, for example, this typically involves manually adjusting the value of at least one component of the bridge to produce a null or zero output when a load known to have the desired characteristic value of impedance is present. This is referred to as nulling or balancing the bridge. Once calibrated, such circuits are operative to detect absolute variations in load impedance from the characteristic impedance, but do not discriminate between or provide any indication concerning different types or causes of impedance variations.
In addition, the known circuits and instruments, particularly those employing directional couplers, generally respond undesirably to variations in the frequency of the forward RF signal. As a result, the frequency of the forward signal must generally be maintained within a narrow bandwidth if such circuits and instruments are to provide accurate power measurements. This limitation has the undesirable effect of restricting the use of directional couplers to relatively narrow bandwith signals. Variations in the magnitude of the forward RF signal also affect measurements of the reflected RF signal by such circuits and instruments and may be misinterpreted by such circuits and instruments as variations in load impedance. This effect is particularly undesirable in certain applications, such as those described below, wherein data is transmitted by the load by intentionally modulating its own impedance.
In certain biomedical applications, for example, an external source communicates RF power and/or data signals to an implanted prosthetic or telemetry device (the load) through inductively coupled coils present in the source and load. In some applications, the load may reflect telemetry, status, or other data signals back to the source by impedance modulating the load impedance as a function of the data to be transmitted. For example, the load may include a real or reactive component the value of which is continuously varied as a function of the data to be transmitted, e.g., ECG telemetry data, in order to reflect a portion of the transmitted signal back to the source with magnitude or phase variations representative of the load data.
Alternatively, the load may include means to switch a real or reactive component into or out of circuit with the load in order to reflect binary encoded data pulses to the source. For example, in one application which involves a portable data storage and communication system, a portable, hand-held source of RF power and digital data signals communicates via inductively coupled coils with a miniature load in the form of a data storage device affixed to a patient's wristband, a test tube, a credit card, or other portable object. The data storage device receives and stores the digital data signals transmitted by the source and communicates stored digital data to the source by rapidly switching a resistor in and out of circuit as a function of the state of the stored data signals to reflect digital data pulses to the source. The source detects the reflected data pulses and recovers the transmitted data. This application is fully described and claimed in co-pending U.S. patent application Ser. No. 818,469 filed Jan. 13, 1986 which is assigned to the assignee of this application.
In applications such as those described above where data is transmitted via modulation of the load impedance, it is critically important to discriminate between and separate intentional load impedance modulation used to transmit data from load impedance variations due to other causes. For example, it has been found in the case of implanted telemetry devices that shifting of the device in the body over time alters the relative positions of the inductive coils. It has also been found that hand-held RF source generally cannot be held steady enough to prevent substantial variations in the relative positions of the source and load coils. In both cases, relative displacement of the coils causes the inductive coupling coefficient between the coils and hence the load impedance presented to the source to change or vary. The applicant has discovered that impedance variations due to such coil displacement, for example, are generally relatively low frequency, long term basal variations in comparison to the rapid, high frequency variations due to intentional impedance modulation.
Previous attempts to overcome communications difficulties caused by the displacement of the inductively coupled coils have included adjusting the coupling coefficient by stagger tuning the coils and adjusting the output power level using closed loop feedback control. See, for example, A Wide-Band Inductive Transdermal Power and Data Link With Coupling Insensitive Gain, Galbraith et al., IEEE Transactions on Biomedical Engineering, Vol. BME-34, No. 4, Apr. 1987; and Control External Powering of Miniaturized Chronically Implanted Biotelemetry Devices, Kadefors, IEEE Transactions on Biomedical Engineering, Vol. BME-23, No. 2, Mar. 1976. The former solution, however, is impracticable with monolithic integrated circuit loads, because of the difficulty and expense of fabricating reactive components in the integrated circuit having the precise values necessary to effect the proper tuning. The latter solution employs a separate feedback transmission signal to adjust the forward power level as a function of the level of the signal received in the load rather than detecting variations in the reflected signal in the primary communications channel.
Thus, there is a need which is not satisfied by known circuits and instruments for an apparatus, hereinafter referred to as a reflectometer, that detects and is responsive to the RF signal reflected by a load and that is insensitive to frequency and magnitude variations in the transmitted or forward RF signal. There is a need for such a reflectometer that is operative over a range of load impedances and that has self-balancing means operative to automatically achieve and maintain a balanced or null output condition for any value of load impedance in that range. Such an apparatus will hereinafter be referred to as a self-balancing reflectometer. There is a need for such a reflectometer that provides simplified means to generate both accurate sign and magnitude information for variations in load impedance as indicated by variations in the reflected RF signal. There is also a need for such a reflectometer that automatically self-balances in response to certain long term basal load impedance variations due to factors such as coil displacement which are unrelated to data transmission and that is capable of simultaneously detecting intentional high frequency load impedance modulation used to transmit load data and providing a detectable output indicative thereof. There is in addition a need for such a reflectometer that is selectably responsive to either the real or imaginary component of load impedance.
A self-balancing reflectometer having the foregoing features and advantages will find use not only in conventional RF communications applications but also in electrical load testing and measurement applications, as well as other data communications applications which employ load impedance modulation techniques. It is therefore an object of the present invention to provide a self-balancing reflectometer apparatus having the foregoing features and advantages as well as others which will be apparent to those skilled in the art.